Delta bonds and rotational barriers in 4d and 5d metal-porphyrin dimers

Abstract

This review summarizes the contribution made to the field of delta bonds by Collman and co-workers. The delta (σ) bond is unique to metal-metal multipty bonded systems and has been the subject of several reviews (F. A. Cotton and R.A. Walton,Multiple Bomls Between Metal Atoms (Oxford University Press, New York, 1993), and references therein). Our contribution to this field is the measurement of the barrier to rotation about the axis of the metal-metal bond for a series of Mo and W porphyrin dimers. (J. P. Collman and L K. Woo (1984).Proc. Natl. Acad. Sci. USA 81, 2592-2596; J. P. Collman, J. M. Garner, R.T. Hembre, and Y. Ha (i992).J. Am. Chem. Soc. 114, 1292–1301). The barriers to rotation for the porphyrin dimers are as follows: ΔG: rot,=9.8 to 10.7±0.5 kcal/mol for a series of [M(OEP-X)]2 dimers (X=CHO, NCO), ΔG: rot,= 10.5±0.5 kcal/mol for [Mo(TOEP)]2 and ΔG: rot= 12.9∓0.1 kcal/mol for [W(TOEP)], Each of these barriers reflects both the strength of the delta bond and steric interactions. The steric component is estimated to be small ( <5 kcal/mol); hence the barrier can be used as an estimate for the delta bond strength. These studies constitute the firstdirect experimental probe of the strength of delta bonds. These data for the rotational barriers in Mo and W dimers are of the same order of magnitude as the estimate of the 6 bond strength obtained using a generalized valenee bond treatment (6±3 kcal/mol) (D. C. Smith and W. A. Goddard 1II (1987).J. Am. Chem. Soc. 109, 5580-55831 and are in agreement with the recent estimate of the delta bond strength (⊃10kcal/mol) (M.D. Hopkins, H.B. Gray, and V.M. Miskowski (1987).Polyhedron 6, 705—714). In addition, this study demonstrates that the W (5d)σ bond is significantly stronger (⊃ 20%) than the anatogous Mo (4d)σ bond; this is consistent with the assertion that the greater radial expansion of 5 d orbitals can result in 5d-5d bonds which arestronger eren though they may belonger.